Binary Fe-Free Transition Metal Catalysts for the Oxygen Reduction Reaction

Abstract

Platinum group metal (PGM)-free catalysts with high volumetric activity and low hydrogen peroxide yields are needed as replacements for expensive and scarce platinum-based catalysts for the oxygen reduction reaction (ORR). State-of-the-art PGM-free ORR catalysts are currently synthesized from a variety of carbon-nitrogen-iron (C-N-Fe) precursors. While these C-N-Fe catalysts have shown promising ORR activity and selectivity, a concern exists on the possibility of harmful Fenton (Fe2+/H2O2) and Fenton-like (Fe3+/H2O2) side reactions generating hydroxyl radicals risking the durability of polymer electrolyte fuel cells due to membrane degradation. Therefore, in an effort to mitigate that risk, development of Fe-free catalysts with similar ORR activity to the C-N-Fe system but with alternative transition metal precursors are being pursued. Our previous results have shown improved ORR activity from (CM+PANI)-Co-C catalysts with a half-wave potential of 0.77 V nearing that of Fe-based catalysts [1]. While this improvement is significant, further development is necessary to enhance both activity and selectivity of Fe-free catalysts. In this work, we will exploit the synergism between theoretical and experimental approaches to develop binary Fe-free transition metal ORR catalysts. Earlier results from quantum-chemical models linking atomic scale structure to ORR activity based on density functional theory (DFT) suggest increased ORR activity for mixed-metal-species active site structures in the Fe-Co system [2]. The same effect was observed from the Mn-Co system. A Sabatier principle relationship between ORR activity and single-transition-metal active sites was observed in both theoretical and experimental studies. A similar experimental-theoretical relationship will be developed from our findings of binary Fe-free transition metal sites. Acknowledgement Financial support for this research by DOE-EERE through Fuel Cell Technologies Office is gratefully acknowledged. [1] Martinez, U., Holby, E.F., Dumont, J.H., Chung, H.T., Zelenay, P. “Non-PGM ORR Catalysts Based on Transition Metals Alternative to Iron.” ECS Meeting s, 2016. [2] Holby, E.F., Taylor, C.D. “Activity of N-coordinated multi-metal-atom active site structures for Pt-free oxygen reduction reaction catalysis: Role of *OH ligands,” Scientific Reports, 2015, 5, 9286.